Acoustic wave devices using a piezoelectric thin film resonator have been used as filters and duplexers of wireless devices such as mobile phones. The piezoelectric thin film resonator is structured to have a lower electrode and an upper electrode facing each other across a piezoelectric film.
Rapid spread of wireless systems results in usage of many frequency bands. This requires the skirt characteristics of a filter or a duplexer to be steep. Increasing the Q value of the piezoelectric thin film resonator is one of measures for making the skirt characteristics steep. One of the reasons that the Q value of the piezoelectric thin film resonator deteriorates is leakage of acoustic wave energy from a resonance region to the outside.
Japanese Patent Application Publication No. 2006-109472 (Patent Document 1) discloses that a Q value is improved by providing a circular band to a surface of the lower electrode or the upper electrode.
However, the structure disclosed in Patent Document 1 cannot sufficiently reduce the acoustic wave energy leaking from the resonance region to the outside. Therefore, the improvement of the Q value is insufficient.